Patch from Neil Brown <neilb@cse.unsw.edu.au>

Allow auto-unplugging to work for devices that do it themselves.

Auto-unplugging - and blk_unplug_work in particular - assumes that the device
uses "generic_unplug_device" for unplugging, but some devices don't.  md
crashes.

So blk_unplug_work should use ->unplug_fn